Thermionic cathode



Patented Dec. 13,1938

UNITED STATES PATENT OFFICE THERMIONIC CATHODE Erwin F. Lowry, Wilkinsburg, Pa., assignor to Westinghouse Electric & Manufacturing Company, a corporation of Pennsylvania No Drawing. Application October 28, I926, Serial No. 144,911

11 Claims. (Cl. 250-27.5)

My invention relates to thermionic cathodes emissive surface be fairly extensive and, since a and in particular to cathode materials adapted to large surface presupposes a wire having an apbe coated with electron-emissive oxides. preciable diameter, it is obvious that the material One object of my invention is to provide an used should havea high specific resistance. inexpensive base-metal filament. Some of the popular high-resistance alloys 6 Another object of my invention is to provide a which are extensively used in high-temperature base-metal filament that will not react iniuriwork, such as those known under the trade-marks ously, at elevated temperatures, with the elec- Nichrome, Chromel and Karma, are of the type tron-emissive oxides carried thereby. which form acid oxides. They have, consequent- 1 Another object of my invention is to provide a- 1y, been considered unsuitable for use as a filal0 base-metal-filament material having a relativement material. ly high specific resistance. Other characteristics necessary in a suitable Another object of my invention is to provide a filament material are a reasonable tensile base-metal filament that will be substantially as strength at 1000 C. and suificient ductility to en- I satisfactory in use as the platinum-iridium filaable it to be drawn readily into fine wire not ments heretofore considered necessary as carlarger than 10 mils in diameter. riers for oxide coatings. No single metal, with the possible exception of Another object of my invention is to provide a platinum, and no heretofore employed alloy, exmethod whereby my improved'base-metal alloy v cept platinum-iridium, has embodied all of the so may be prepared from the raw materials. necessary requirements mentioned. It is, accord- It has heretofore been considered necessary to ingly, the principal object of my invention, as employ, as a carrier for an electron emissive oxstated previously, to produce an alloy which will ide coating, a filament made of either platinum or satisfactorily replace platinum or platinum-irida platinum-iridium alloy. ium and which will be very much cheaper to man- It has been commonly accepted as a fact, ufacture. 25 among workers in the thermionic-tube art, that is I have made numerous experiments with alloys to .say, in the making of X-ray tubes, hot cathode of different metals and have discovered that metrectifiers, three-electrode tubes and four elecals of the iron group are the only ones, apparenttrode tubes for radio purposes and the like, that iy. that meet all of the above requirements and metals forming acid oxides are unsuitable for this I have also discovered that these metals are not purpose. It was, consequently, deemed necessary entirely satisfactory in a pure state. to employ one of the so-called noble metals as a v The alloy that I have found to be the most satfilament material, under the impression that, isfactory, comprises nickel, cobalt, iron and tisince they were the only ones which did not retanium. Varying proportions of these elements act with the oxides at an elevated temperature, have been employed with an approximation of their use was imperative. equally good results. I have used alloys con- Among the requirements of a filament matetaining nickel and cobalt in the aggregate of 80 rial may be mentioned freedom from oxidization to 95%, in which the nickel and cobalt were presat high temperatures and high specific resistent in proportions ranging from 95 to 5% nickel ances. The reason back of the first requirement to 5 to 95% cobalt. The combination nickel-cois obvious. The necessity of a high specific rebalt has a higher resistance and is less susceptible sistance arises from the fact that the elements to chemical attack by the oxide-coating mateof substantially all thermionic tubes are relativerials used than either of the metals alone. The ly small in size. In order to secure the necessary remainder of the alloy consists of metals which electron emission from a filament of perhaps only render the nickel-cobalt alloy forgeable and, an inch in length, it is necessary that the 1 1 loss among other things, remove oxides from the albe fairly large. To secure, with a limited curloy. Therefore, I introduce into the alloy from rent consumption, the necessary heating eifect, 5 to 20% of ferrotitanium, ferrovanadium, ferroit is, of course, imperative that the resistance r silicon or ferromolybdenum in an amount sumbe large. It'is also desirable that the electroncient to render the nickel-cobalt forgeable and ductile so that the ingot may be reduced to a fine wire or may be otherwise shaped.

One alloy in particular which I have found extremely satisfactory may be made by combining Parts Nickel 80 Cobalt 20 Iron Titanium /2 the iron and titanium being used in the form of tially as follows:

The above-mentioned proportions of nickel, cobalt, and ferro-titanium are melted together in an electric furnace, care being taken to prevent access of oxygen. After the melt is completed and just before casting, a small amount of magnesium and aluminum is added to serve as de-oxidizing agents. There is also a possibility that the ferro-titanium functions as a de-oxidizing agent as well, and it is within the scope of my invention to use ferro-titanium, ferro-vanadium, silicon, ferro-silicon manganese or form-manganese.

The ingot resulting from the melt is subsequently forged at about 750 C. My experiments seem to indicate that the forging temperature has an important bearing upon the molecular structure of the finished filament, and this temperature should not be permitted to go much above 800 C.

By repeated forging, the ingot is reduced to a convenient size for rolling and swaging', by which latter operation it is reduced-to a diameter of about 50 mils.

It is imperative that, in the rolling and swaging processes, the metal shall be annealed after every second die.

The 50 mil diameter wire is then drawn down to the required size through diamond dies of successively smaller diameters. It is noted that substantially no annealing is necessary after a size of 10 mils has been attained. I have found that the finished filaments have resistances of approximately 40 ohms per milinch as compared with 22 ohms per mil-inch for a 10% platinum-iridium alloy. This feature is of distinct advantage because it practically doubles the cross-section of a filament having the same electrical and thermionic characteristics as a filament of a given size composed of platinumiridium.

This high specific resistance has a further advantage in that it will permit filaments of the oxide-coated type to be made having certain filament voltage-current relations which have been heretofore commercially impracticable when employing platinum-iridium. For example, a filament .75 inch long may be made from my improved nickel-cobalt alloy which will reach the proper operating temperature at .3 volt and .06 ampere. This feature will enable the thoriated tungsten filament in tubes,*such as the small UV-199, to be replaced by oxide-coated filaments.

My nickel-cobalt alloys have an extremely high tensile strength when cold, a tensile strength of over 100,000 lbs. per sq. in. not being unusual. The utilization of a material of such strength greatly decreases the likelihood of breakage during manufacture.

Furthermore, it has been found that filaments made of certain of these nickel-cobalt alloys will have the very advantageous property that, when heated to 1000 C., they will stretch as much as 10 to 12% of their length before breaking. This stretching, when heated, obviates the necessity for a critical adjustment of a spring support to prevent sag caused by thermal expansion. The take-up of the spring must be only slightly greater than the thermal expansion expected, and the rather critical adjustment of the filament hooks to take care of the expansion that takes place in platinum-iridium filaments is unnecessary. For this reason, a filament comprising my new alloy will be non-microphonic since it will stretch, under tension of the springs, until the tension is relieved at the operating temperature. The limits thus set for the spring take-up are, accordingly, not less than the thermal expansion of the filament and not more than 10% of the filament length.

I have found also that cobalt is a very satisfactory material with which to electroplate a filament,..otherwise unsatisfactory, as a carrier 'for emissive oxides. The cobalt, in this case, serves to protect the oxide coating from the harmful influence of the base-metal-filament. For this purpose, the cobalt may be applied to the filament material as a sleeve instead of by electroplating and both metals drawn down and rolled to the required size together.

No special technique is required in applying the oxide coatings to filaments of any of the herein described alloys or to filaments which have previously been coated with pure cobalt, although a mixture of the carbonates of barium and strontium suspended in ethyl-alcohol is suggested as being the most satisfactory coating compound.

Where the expression cobalt-nickel occurs in the claims, it signifies that the proportion of cobalt to nickel lies within the limits specified previously, namely 95 to nickel and 5 to 95% cobalt. Where the claims state that a core or alloy or filament comprise. particular elements, it means that no other substances are present in such amount as would substantially affect the properties of the resulting material.

Although I have described herein certain specific embodiments of my invention, other modifications will be apparent to those skilled in the art. My invention, therefore, is. not to be limited except insofar as is necessitated by the prior art and as indicated in the appended claims.

I claim as my invention:

1. In combination with an evacuated container, an electrode comprising a base metal composed oi an alloy comprising cobalt-nickel and titanium and a layer of electron-emissive material on said base metal, said cobalt nickel be, ing from 80 per cent to 95 percent of .said alloy.

2. In combination with an evacuated container, an electrode comprising a base metal composed of an alloy consisting of a compound of cobalt and nickel of which neither constituent is less than 5% and an element of the group consisting of titanium, vanadium. and manganese,-

and a layer of electron-'emissive material on said base metal, said compound being from percent to percent of said alloy.

3. A cathode element for an electron discharge device composed of an alloy comprising a base selected from the group comprising nickel and cobalt and constituting at least 90% of the alloy, and an added quantity of substance taken from thegroup comprising silicon, titanium, vanadium and zirconium, the added quantity including at least silicon and titanium, the silicon constituting at least of 1% of the alloy, and the titanium constituting at least of 1% of the alloy.

4. A cathode element for an electron discharge device composed of an alloy comprising a base selected from the group comprising nickel and cobalt and constituting at least 90% of the alloy, and an added quantity of substance taken from the group including silicon, titanium, vanadium and zirconium, the added quantity including at least silicon and vanadium, the silicon constituting at least of 1% of the alloy, and vanadium constituting at least /4 of 1% of the alloy.

5. In a hermetically sealed evacuated tube. a metallic cathode element exposed to the rarefied atmosphere within said tube and composed of a malleable and ductile deoxidized and degasified alloy comprising a metal having the characteristics common to nickel and cobalt and containing silicon, and low in carbon.

8. In a hermetically sealed evacuated tube, a metallic cathode element exposed to the rarefied atmosphere within said tube and composed oi. a malleable and ductile alloy comprising a metal having the characteristics common to nickel and cobalt and containing a deoxidizer in excess of the amount required to eflect substantial deoxidation so as to eflect also substantial degasiflca tion or the alloy, the unoxidized excess remaining in the alloy as a constituent thereof, said alloy being low in carbon.

'1. In a hermetically sealed evacuated tube, a metallic cathode element exposed to the rarefied atmosphere within said tube and composed of a malleable and ductile alloy comprising a metal having the characteristics common to nickel and cobalt and containing silicon in excess of the amount required to effect substantial deoxidation so as to effect also substantial degaslflcation of the alloy, the unoxidized excess of the silicon remaining in the alloy as a constituent thereof, said alloy being low in carbon.

8. In combination with an evacuated container, an electron-emissive cathode having a core comprising cobalt, nickel, titanium and iron, said cobalt and nickel being from 80% to 95% and said titanium and iron being from 5% to 20% of said core.

9. In combination with an evacuated container, an electrode comprising a base metal composed of an alloy comprising cobalt, nickel, iron and titanium and a layer oifelectron-emissive material on said base metal, said cobalt and nickel being from 80% to 95% and said iron and titanium being from 5 to 20% of said alloy.

10. In combination with an evacuated container, an electrode comprising a base metal composed of an alloy comprising cobalt, nickel and iron and an element of the group consisting of titanium, vanadium and manganese, and a layer of electron-emissive material on said base metal, said cobalt and nickel being from 80% to 95% of said alloy and said iron and element being from 5% to 20% of said alloy.

11. A cathode comprising a base metal consisting of 80% to 95% of a compound of cobalt and nickel of which neither constituent is less than 5% and the remaining 20% to 5% a deoxidizer consisting of one or more of the metals iron, titanium, vanadium or silicon and an electronemissive coating on said base metal.

ERWIN F. LOWRY. 

